JP2018093429A - Light transmitter module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light transmitter module operable to combine three or more light signals different in intensity in a multiplexer to produce PAM signals, in which the beat noise accompanying the difference in wavelength, and the influence of wavelength distribution can be reduced.SOLUTION: A light transmitter module 1 comprises: first to third light signal sources 11 to 13 which output light signals binary intensity modulated by amplitudes different from each other; and a combination part 20 which combines a plurality of input light signals into one output light signal. The combination part 20 has: a wavelength multiplexer 21 which performs wavelength multiplexing of a plurality of input light signals having wavelengths different from each other while holding polarization states; and a polarized wave multiplexer 22 which performs polarized wave multiplexing of a pair of input light signals having polarization states orthogonal to each other, and which combines input light signals from the first to third light signal sources 11 to 13 to produce PAM8 signals.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission module, and more particularly to a technique for generating a multilevel intensity modulated optical signal having eight or more values.

  In recent years, the amount of Internet communication has been increasing due to the explosive spread of portable data communication devices such as smartphones. Most of high-speed and large-capacity information transmission corresponding to this increase in communication volume is realized by optical fiber transmission. From a distance of about several hundred meters connecting between devices to several km to several tens km connecting inside and outside the data center. It is desired to increase the speed of optical fiber transmission lines. A representative of the current large-capacity transmission standard is 100 Gigabit Ethernet (100 GbE) (Ethernet is a registered trademark) defined as IEEE 802.3ba in 2010. Also, standardization work for 400 Gigabit Ethernet (400 GbE) has been started to cope with further increase in traffic.

  In such high-speed and large-capacity communication, in order to overcome the limitations on the operating speed of laser light sources and optical modulators, the application of multi-level modulation is being considered in place of conventional binary modulation. In binary modulation, the logic “1” and logic “0” of the binary information signal to be transmitted correspond to the maximum and minimum light intensity, respectively. On the other hand, in multi-level modulation, a multi-level information signal to be transmitted is made to correspond to a multi-level using an intermediate level between the maximum and minimum light intensity. For example, in 400 GbE, PAM4 which is quaternary pulse amplitude modulation is used.

  Regarding PAM, in Patent Document 1 below, a digital / analog converter (DAC) is used to convert a digital signal into a multi-level voltage signal, and the voltage signal is used as a modulation signal for an optical modulator. A technique that enables multi-level light intensity modulation by driving is disclosed. Further, in Patent Document 2 below, as a technique for realizing multilevel light intensity modulation without using a DAC, two optical signals that are binary intensity modulated with different amplitudes are combined by a multiplexer to combine them with PAM4. A scheme for obtaining a signal is shown.

Japanese Patent Laying-Open No. 2015-207803 JP 63-5633 A

  In the future, in order to further increase the amount of communication, it may be necessary to put into practical use a multi-level modulation scheme such as an 8-level PAM8, a 16-level PAM16, and the like.

  In PAM communication, it is desirable that the level intervals of the optical output signals be uniform in order to improve the amplitude detection resolution. However, in the method of generating a multilevel optical output signal by inputting a multilevel modulation signal to the optical modulator as in the above-described method using the DAC, the modulation signal is set so that the output levels are equally spaced. It is not always easy to accurately control the multi-value level. In particular, a Mach-Zehnder (MZ) type modulator, an electro absorption (EA) modulator, or an electro-optic modulator such as a semiconductor laser, which is generally used for optical fiber communication, has nonlinear modulation characteristics. Have For this reason, the level interval of the electrical signal as the modulation signal is basically unequal, and there is a problem that the modulation signal generation circuit that suitably equalizes the level of the optical output signal becomes complicated or large. For example, in the case of multilevel PAM modulation using a DAC, it is necessary to use a DAC having a large effective number of bits (ENOB) in order to compensate for this nonlinearity. However, as the transmission speed increases, the processing load of the DAC also increases and the ENOB becomes smaller. Therefore, it can be a problem to construct a DAC having sufficient ENOB for nonlinearity compensation.

  In this regard, in the method of generating a PAM signal by combining a plurality of optical signals having different intensities with a multiplexer, each optical signal to be combined has a binary level and has only one level interval, so that the above-described modulation characteristics are obtained. Therefore, it is relatively easy to control multilevel levels in the optical output signal. On the other hand, in this method, it is ideal that the wavelengths of multiple optical signals to be combined are the same, but this is difficult in practice, and beat noise that occurs due to wavelength differences and wavelength dispersion during fiber transmission Can affect the code quality (transmission quality).

  Here, the number of optical signals to be combined increases with the number of multilevel levels. For example, PAM8 requires three optical signals having different intensities, and PAM16 requires four optical signals. When the number of optical signals to be combined is large, the influence due to the above-described difference in wavelength can be large, so that it is a problem to deal with the influence.

  This problem will be further described. Regarding beat noise in wavelength multiplexing, it is possible to avoid a decrease in code quality of a PAM signal by providing a certain wavelength difference between optical signals to be multiplexed. However, if an attempt is made to provide a wavelength difference that can avoid the influence of beat noise in any combination of multiple optical signals, the range in which the wavelengths of these optical signals are set increases as the number of optical signals increases. There arises a problem that the influence of the wavelength dispersion becomes large. In other words, the chromatic dispersion is larger when generating the PAM8 signal from the three-wavelength optical signal than when generating the PAM4 signal by combining the two-wavelength optical signals, and generating the PAM16 from the four-wavelength optical signal. In such a case, the chromatic dispersion tends to increase further.

  Note that beat noise does not occur in orthogonal polarization multiplexing, but three or more optical signals cannot be in a state where their polarizations are orthogonal to each other. It cannot be generated.

  We have found that the above-mentioned problem exists regarding the generation of a PAM signal having eight or more values by multiplexing, and have conceived the present invention. That is, an object of the present invention is to solve the above-described problems associated with wavelength differences in an optical transmission module that generates a PAM signal by combining three or more optical signals having different intensities with a multiplexer.

(1) An optical transmission module according to the present invention includes first to nth optical signal sources (n is a natural number of 3 or more) that outputs optical signals that are binary intensity modulated with different amplitudes, and a plurality of optical signals. A combining unit that combines the input optical signals to generate one output optical signal, and the combining unit wavelength-multiplexes the plurality of input optical signals having different wavelengths while maintaining the polarization state. A multiplexer and a polarization multiplexer that polarizes and combines a pair of input optical signals having polarization states orthogonal to each other, and receives the input optical signals from the first to n-th optical signal sources. By combining, an output optical signal that is pulse amplitude modulated to 2 ⁿ level is generated.

を満たす構成とすることが好適である。 (2) In the optical transmission module according to (1) above, the wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂₎ of the optical signals of the two optical signal sources that are combined with each other by the wavelength multiplexer. ), Where the speed of light is C ₀ and the modulation rate of the optical signal is BR.

It is preferable that the configuration satisfies the above.

(3) In the optical transmission module described in (1) and (2) above, the upper limit wavelength and the lower limit wavelength of the first to nth optical signal sources are λ _M + Δλ and λ _M −Δλ, respectively, and the optical transmission line is The difference in group delay time between the light of the upper limit wavelength and the light of the lower limit wavelength in the output optical signal of the optical transmission module after the distance L transmission is Δτ, and the upper limit wavelength and the lower limit wavelength are:
Δτ = 2LSΔλ (λ ₀ −λ _M )
(Where λ ₀ is the zero-dispersion wavelength of the optical transmission line, and S is the slope of the chromatic dispersion coefficient of the optical transmission line with respect to the wavelength.) Δτ given by a predetermined distance L It is preferable that the configuration satisfies the condition that the value is equal to or less than a predetermined allowable value.

  (4) In the optical transmission module according to the above (1) to (3), the n is 3, and the first optical signal source and the second optical signal source have different wavelengths and the same polarization state. The third optical signal source outputs an optical signal in a polarization state orthogonal to the first optical signal source and the second optical signal source, and the wavelength multiplexer The optical signal of the first optical signal source and the optical signal of the second optical signal source are combined, and the polarization multiplexer includes the output optical signal of the wavelength multiplexer and the third optical signal source. It can be set as the structure which multiplexes with an optical signal.

  (5) In the optical transmission module according to (1) to (3), n is 3, and the first optical signal source and the second optical signal source have different wavelengths and the same polarization state. The third optical signal source outputs an optical signal in a polarization state orthogonal to the first optical signal source and the second optical signal source, and the polarization multiplexer is The optical signal of the second optical signal source and the optical signal of the third optical signal source are multiplexed, and the wavelength multiplexer includes the output optical signal of the polarization multiplexer and the first optical signal source. The optical signal can be combined.

  (6) In the optical transmission module according to (4) and (5) above, the relative intensities of the components of the optical signal of each optical signal source in the output optical signal of the combining unit are 1, 2, and 4. be able to.

  (7) In the optical transmission module according to (1) to (3), n is 4, the first optical signal source and the second optical signal source have different wavelengths and a first polarization. An optical signal having a state, wherein the third optical signal source and the fourth optical signal source output optical signals having different wavelengths and a second polarization state orthogonal to the first polarization state. The wavelength multiplexer includes a first wavelength multiplexer that multiplexes the optical signal of the first optical signal source and the optical signal of the second optical signal source, and the third optical signal source. A second wavelength multiplexer that multiplexes an optical signal and an optical signal of the fourth optical signal source, and the polarization multiplexer includes an output optical signal of the first wavelength multiplexer; The output optical signal of the second wavelength multiplexer can be combined.

  (8) In the optical transmission module according to (1) to (3), n is 4, and the first optical signal source and the third optical signal source have different wavelengths and a first polarization. An optical signal having a state, wherein the second optical signal source and the fourth optical signal source output optical signals having different wavelengths and a second polarization state orthogonal to the first polarization state. The polarization multiplexer outputs a first polarization multiplexer that combines the optical signal of the first optical signal source and the optical signal of the second optical signal source, and the third optical signal. A second polarization multiplexer that combines the optical signal of the source and the optical signal of the fourth optical signal source, and the wavelength multiplexer is an output of the first polarization multiplexer The optical signal and the output optical signal of the second polarization multiplexer can be combined.

  (9) In the optical transmission module according to the above (1) to (3), n is 4, the first to third optical signal sources output optical signals having different wavelengths, and the fourth The optical signal source outputs an optical signal having a polarization state orthogonal to the polarization state of the third optical signal source, and the polarization multiplexer includes the optical signal of the third optical signal source and the fourth optical signal source. A first wavelength multiplexer that multiplexes the optical signal of the first optical signal source and the optical signal of the second optical signal source; And a second wavelength multiplexer that multiplexes the output optical signal of the polarization multiplexer and the output optical signal of the first wavelength multiplexer.

  (10) In the optical transmission module described in (1) to (9) above, one of the two optical signal sources that outputs optical signals in the polarization state orthogonal to each other outputs the output optical signal to the other It is possible to employ a configuration having polarization rotation means for making the polarization state orthogonal to the optical signal source.

  (11) In the optical transmission module described in (1) to (10) above, the optical signal source may be a semiconductor optical device having a distributed feedback laser and an electroabsorption modulator.

(12) In the optical transmission module described in (4) to (6) above, when the modulation rate BR of the optical signal is 28 gigabaud (GBaud) and the transmission distance L in the optical transmission line is 2 kilometers (km) Further, an allowable value of the group delay time difference Δτ between the upper limit wavelength and the lower limit wavelength of the optical signal is set to 1/5 of the reciprocal of the modulation rate BR, and the optical transmission line has a zero dispersion wavelength λ _0. Each of the first to third optical signal sources is 1310 nanometers (nm), and the slope S of the wavelength dispersion coefficient with respect to the wavelength is 8.7 × 10 ⁻¹⁴ seconds / km · nm ² . The wavelengths λ ₁ , λ ₂ , λ ₃ are
1300.9 nm ≦ λ ₁ ≦ 1304.84 nm
1305.16 nm ≦ λ ₂ ≦ 1309.1 nm
1300.9 nm ≦ λ ₃ ≦ 1309.1 nm
It can be set as the structure which satisfy | fills.

  According to the present invention, an optical transmission module capable of generating a PAM signal having a suitable code quality while having a multi-level higher than that of PAM4 is obtained.

It is a schematic diagram which shows the structure of the outline of the optical transmission module which concerns on the 1st Embodiment of this invention. It is a schematic diagram of the outline of the optical transmission module showing the structural example of the optical signal source. It is a schematic diagram of the vertical cross-sectional structure of a modulator integrated semiconductor laser element. It is a graph which shows the change of (DELTA) (lambda) when changing a center frequency. It is a schematic diagram which shows the structure of the outline of the optical transmission module which concerns on the 2nd Embodiment of this invention. It is a schematic diagram which shows the structure of the outline of the optical transmission module which concerns on the 3rd Embodiment of this invention. It is a schematic diagram which shows the structure of the outline of the optical transmission module which concerns on the 4th Embodiment of this invention. It is a schematic diagram which shows the structure of the outline of the optical transmission module which concerns on the 5th Embodiment of this invention.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

[First Embodiment]
The optical transmission module 1 according to the first embodiment is an apparatus used for optical communication using, for example, an optical fiber as a transmission path, and generates a PAM8 signal from three optical signals and outputs it to the optical transmission path. FIG. 1 is a schematic diagram showing a schematic configuration of the optical transmission module 1. The optical transmission module 1 includes a first optical signal source 11, a second optical signal source 12, a third optical signal source 13, and a combining unit 20. The synthesizer 20 includes a wavelength multiplexer 21 and a polarization multiplexer 22, and synthesizes input optical signals from the optical signal sources 11, 12, and 13 to generate one output optical signal _SOUT . In order to transmit optical signals between components in the optical transmission module 1, the discrete components are connected by optical fibers, and the integrated components are connected by waveguides. The connection between the discrete components is not limited to this, and a spatial optical system using lenses, mirrors, or the like instead of the optical fiber / waveguide or a waveguide formed entirely of a semiconductor may be used.

The optical signal sources 11, 12, and 13 generate optical signals that are binary intensity modulated with different amplitudes and output the optical signals to the combining unit 20. The wavelengths of the optical signals of the optical signal sources 11, 12, and 13 are represented as λ ₁ , λ ₂ , and λ ₃ , respectively. The optical signal sources 11 and 12 output optical signals having the same polarization state (for convenience, the TE mode), and the optical signal source 13 has a polarization state orthogonal to the optical signal sources 11 and 12 (for convenience, TM mode) is output.

  The wavelength multiplexer 21 multiplexes a plurality of input optical signals having different wavelengths while maintaining the polarization state. On the other hand, the polarization multiplexer 22 combines a pair of input optical signals having polarization states orthogonal to each other. In the present embodiment, the wavelength multiplexer 21 wavelength-multiplexes the TE mode optical signals from the optical signal sources 11 and 12 and generates a TE mode optical signal including these two optical signals as components. The polarization multiplexer 22 polarization-multiplexes the optical signal generated by the wavelength multiplexer 21 and the optical signal from the optical signal source 13, and converts the optical signal from the wavelength multiplexer 21 into a TE mode component. An optical signal including the optical signal from the optical signal source 13 as a TM mode component is generated.

The synthesizer 20 outputs the optical signal generated by the polarization multiplexer 22 as S _OUT . The amplitudes of the optical signals of the optical signal sources 11, 12 and 13 are set so that the intensity ratio of the optical signal components of the respective optical signal sources in the output optical signal S _OUT of the combining unit 20 is 1: 2: 4. Thereby, the optical transmission module 1 generates a PAM8 signal. The optical signal generated by the optical transmission module 1 is output to the optical fiber.

  FIG. 2 is a schematic diagram of an outline of the optical transmission module 1 showing a configuration example of the optical signal sources 11, 12, and 13. The optical signal sources 11, 12, and 13 have modulator integrated semiconductor laser elements 31, 32, and 33, respectively. Each of the modulator integrated semiconductor laser elements 31, 32, and 33 is an element in which a modulator is monolithically integrated in front of the semiconductor laser. The modulator modulates the intensity of the optical signal output from the semiconductor laser based on the modulation signal. A modulated optical signal is generated. In this embodiment, the modulator integrated semiconductor laser device 31 includes a distributed feedback (DFB) laser 31a as a semiconductor laser and an EA modulator 31b as a modulator. Similarly, the modulator integrated semiconductor laser element 32 includes a DFB laser 32a and an EA modulator 32b, and the modulator integrated semiconductor laser element 33 includes a DFB laser 33a and an EA modulator 33b.

  FIG. 3 is a diagram schematically showing the structure of the modulator integrated semiconductor laser elements 31 to 33. As an example, a vertical sectional structure along the optical signal path of the element 31 is shown. In FIG. 3, the rightward direction is the light output emission direction, the DFB laser 31a is disposed on the left side of the element 31, the EA modulator 31b is disposed on the right side, and the connection waveguide 40 is provided therebetween. An electrode 41 of the DFB laser 31a and an electrode 42 of the EA modulator 31b are disposed on the upper surface of the modulator integrated semiconductor laser element 31, and a common electrode 43 of the DFB laser 31a and the EA modulator 31b is disposed on the lower surface. .

Usually, the semiconductor laser of the modulator integrated type semiconductor laser element that outputs a single wavelength light such as the DFB laser of this embodiment or the DBR laser is used. In this respect, the DFB lasers 31a, 32a, and 33a also oscillate in a single mode, and generate light whose wavelengths are basically maintained at λ ₁ , λ ₂ , and λ ₃ described above. For example, λ ₁ , λ ₂ , and λ ₃ are set to wavelengths in the 1.3 micrometer (μm) band where transmission loss in the optical fiber is relatively small.

  The optical signal sources 11, 12, and 13 have a laser drive circuit 50 shown in FIG. 2 as a supply source of drive power to the DFB lasers 31a, 32a, and 33a. In FIG. 2, for convenience of illustration, a laser drive circuit 50 is drawn outside the optical signal sources 11, 12 and 13. The laser drive circuit 50 is connected to the DFB lasers 31 a, 32 a, and 33 a, and allows a current to flow between the electrodes 41 and 43. As a result, the semiconductor laser element emits light by injecting a current into the active layer.

The amplitude (intensity) of the light output of the semiconductor laser element changes according to the injection current. The magnitude of the current supplied from the laser driving circuit 50 to each of the DFB lasers 31a, 32a, and 33a is set separately. Thereby, as described above, the light of each optical signal source in the output optical signal S _OUT of the combining unit 20 The intensity ratio of the signal components is set to be 1: 2: 4. The intensities of the DFB lasers 31a, 32a, and 33a are set in consideration of the loss in the optical components and the optical transmission path up to the output end of the combining unit 20, and according to the loss that can be different for each optical transmission module 1. It is preferable that the adjustment is possible. In this embodiment, an independent light source (semiconductor laser) is used for each optical signal source. However, the present invention is not limited to this, and the optical output of one light source may be branched and input to each EA modulator. However, providing an independent light source for each optical signal source has an advantage that the intensity of the optical signal of each optical signal source can be finely adjusted. When one light source is branched, it is necessary to adjust the amount of absorption of the optical signal intensity of each optical signal source by adjusting the voltage applied to the EA modulator instead of the light output of the light source. However, when the voltage applied to the EA modulator is changed, the extinction ratio and the frequency characteristics change at the same time, and it may be difficult to obtain a desired optical signal. By providing an independent light source for each optical signal source as in this embodiment, the optical signal intensity of each optical signal source can be adjusted by adjusting the light output intensity of the light source, and is applied to the EA modulator. It is not necessary to change the voltage greatly, and the influence on the extinction ratio or the like can be suppressed.

  Each of the EA modulators 31b, 32b, and 33b is configured by a semiconductor diode having a pin junction. When a p-type semiconductor is connected to the electrode 42 side, a negative voltage is applied to the electrode 42 to An OFF state in which the optical signal input from the laser is absorbed and extinguished to reduce the optical output is realized. Conversely, if no voltage is applied to the pin junction, the light passes through the EA modulator without being absorbed or extinguished, so that the on state of the optical signal is realized. Compared with the semiconductor laser disclosed in Patent Document 2, the EA modulator can sufficiently reduce the light intensity in the off state. In the case of a semiconductor laser, a binary optical signal is generated depending on whether it is almost illuminated (off state) or sufficiently illuminated (on state). Quality will deteriorate. On the other hand, the off state of the EA modulator can realize an extinction state of 10 dB or more as compared with the on state, and can create a state where the light intensity is smaller than that of the off state of the semiconductor laser. Therefore, when the PAM signal is generated by the EA modulator, there is an advantage that the extinction ratio is larger than that when the PAM signal is generated by the semiconductor laser.

The optical transmission module 1 includes a modulator control circuit 51 that generates control signals for the EA modulators 31b, 32b, and 33b. The modulator control circuit 51 is connected to each of the EA modulators 31b, 32b, and 33b, applies a voltage between the electrodes 42 and 43, and controls on / off of each EA modulator. For example, the modulator control circuit 51 receives a modulation signal corresponding to the information to be transmitted, generates a binary control signal M ₁ for controlling on / off of the EA modulator 31b from the modulation signal, and similarly performs EA modulation. Binary control signals M ₂ and M ₃ are generated for the devices 32b and 33b, respectively. For example, the EA modulators 31b, 32b, and 33b have a common configuration, and the modulator control circuit 51 drives each EA modulator under common driving conditions. That is, the voltages applied when the EA modulators 31b, 32b, and 33b are on are common, and the voltages applied when they are off are common.

In PAM8, the amount of information carried on one symbol is 3 bits, and the modulator control circuit 51 assigns 3 bits corresponding to 1 symbol to each optical signal source one bit at a time in the modulated signal. The optical signal modulated by the EA modulator based on the bit is output, and the optical signals of the three optical signal sources are combined into the PAM8 signal by the combining unit 20. Here, it is necessary to synthesize three optical signals each corresponding to 1 bit into the same symbol, and in order to realize this, generation of control signals M _{1 to} M ₃ for the EA modulator by the modulator control circuit 51 The timing is adjusted according to the propagation time of light from each EA modulator to the output terminal of the combining unit 20. In order to facilitate the adjustment, it is preferable that the light propagation time from each EA modulator to the output end of the combining unit 20 is approximately the same.

  Further, in order to facilitate the design and manufacture of the element, it is convenient that the modulator integrated semiconductor laser elements 31, 32, and 33 basically have a common configuration except for the wavelength. Therefore, in this embodiment, the modulator integrated semiconductor laser elements of the optical signal sources 11, 12, and 13 all have a common configuration for outputting TE mode light, while the optical signal source 13 includes a polarization rotation plate. 52 is provided. The polarization rotation plate 52 is a half-wave plate and rotates the polarization state of the output light by 90 degrees with respect to the input light. The optical signal output from the modulator integrated semiconductor laser element of the optical signal source 13 is input to the polarization rotation plate 52, and the optical signal whose polarization state is changed to the TM mode by the polarization rotation plate 52 is the optical signal. Output from the source 13. In order to change the polarization state, not only the polarization rotation plate but also other polarization rotation means may be used. For example, the polarization state may be rotated by applying stress to the optical fiber.

  The configuration in which the optical transmission module 1 generates the optical signal of the PAM 8 from the modulation signal for 3 bits corresponding to one symbol has already been briefly described. The features of the present invention in the configuration will be further described below.

  As described above, the 3-bit modulation signal is decomposed into bits, and different bits are input to the EA modulators 31b, 32b, and 33b of the optical signal sources 11, 12, and 13, respectively. Incidentally, the modulation signal is an NRZ (Non Return to Zero) system.

Further, as described above, the relative magnitudes of the intensity of the optical signal components of the optical signal sources 11, 12, and 13 in the output optical signal S _OUT of the combining unit 20 are set to be 1, 2 and 4. . Hereinafter, the intensities of the optical signal components of the optical signal sources 11, 12, and 13 in the output optical signal S _OUT are expressed as P ₁ , P ₂ , and P ₃ .

Here, the value of P ₃ when the value of one bit of the modulation signal input to the EA modulator 33b of the optical signal source 13 as the control signal M ₃ is "1" as the reference value P _0, of the modulated signal The value of P ₃ when the value is “0” is set to 0. For example, the values of P ₁ and P ₂ when the value of the 1-bit modulation signal input as the control signals M ₁ and M _{2 to} the EA modulators 31 b and 32 b of the optical signal sources 11 and 12 is “1”, respectively. Are 4P ₀ and 2P _0, and the values of P ₁ and P ₂ when the value of the modulation signal is “0” are 0.

Table 1 is a table showing a relationship between combinations of values of 1-bit modulation signals input to the EA modulators 31b, 32b, and 33b and the intensity of the output optical signal _SOUT of the combining unit 20 with respect to the combinations. . As shown in Table 1, it is possible to generate a PAM8 signal by synthesizing 1-bit NRZ signals output from three optical signal sources having different sizes.

  One feature of the present invention is that when a plurality of optical signals are combined to sequentially integrate three or more optical signals to generate one PAM signal, wavelength combining and polarization combining are used in combination. This is in that only one stage of the stage multiplexing is polarization multiplexing. With regard to this feature, the optical transmission module 1 has a structure in which the first stage of the two stages of multiplexing is wavelength multiplexing and the second stage is polarization multiplexing. Thus, the combined use of wavelength multiplexing and polarization multiplexing reduces the number of optical signals to be wavelength combined. As a result, the number of wavelengths of the optical signal can be reduced, or the range in which the wavelength of the optical signal is set can be reduced, and an effect of suppressing chromatic dispersion can be obtained while avoiding the influence of beat noise.

The wavelengths λ ₁ , λ ₂ , and λ ₃ of the optical signal combined by the optical transmission module 1 will be described in more detail.

Since the optical signal from the optical signal source 11 and the optical signal from the optical signal source 12 are wavelength-multiplexed by the wavelength multiplexer 21, these wavelengths λ ₁ and λ ₂ are λ ₁ to avoid the influence of beat noise. ≠ λ ₂ is set. Here, λ ₁ <λ ₂ and the modulation rate of the optical output signal S _OUT of the optical transmission module 1 is BR (unit: Baud). The condition under which beat noise does not affect the quality of the optical output signal modulated at the modulation rate BR when optical signals of wavelengths λ ₁ and λ ₂ are combined by the wavelength multiplexer 21 is expressed by the following equation. . C ₀ is the speed of light, which is 3 × 10 ⁸ meters / second (m / s).

The left side of the equation (1) is the beat frequency when the optical signals having the wavelengths λ ₁ and λ ₂ are combined. Equation (1) is based on the fact that if the beat frequency is twice or more the modulation rate BR, the transmission signal is not affected by the response speed limitation on the receiving side or the high-frequency cut filter that is normally inserted. When the central wavelengths of the wavelengths λ ₁ and λ ₂ are λ _C and λ ₁ and λ ₂ are set as λ ₁ = λ _C −Δλ and λ ₂ = λ _C + Δλ, the following expression is obtained from the expression (1).

  When the equal sign is obtained in equation (2), the following equation is established.

For example, assuming that the modulation rate BR is 28 GBaud and λ _C is 1305.0 nm, Δλ is 0.16 nm from the equation (3). That is, if the wavelength difference between two optical signals to be combined is 0.32 nm or more, it is possible to avoid deterioration of information transmitted by the combined optical signal due to beat noise. In addition, the calculation results in the above calculation and the subsequent calculations are rounded off at appropriate places.

Similarly, when using Equation (3), the wavelength difference (2Δλ) between two optical signals that can be prevented from being deteriorated by beat noise is estimated for the wavelength range of 1260 to 1320 nm that is used in optical signal transmission in the 1300 nm band. , in the case of BR = 28GBaud, λ _C = 1260nm in 2Δλ = 0.296nm, also lambda _C = 1320 nm in 2Δλ = 0.325nm is obtained.

  As described above, in order to avoid the influence of beat noise in wavelength multiplexing, a wavelength difference may be provided between two optical signals based on the formula (1) or (2). Further, in order to reduce the influence of chromatic dispersion, it is preferable that the wavelength difference is small, but the lower limit is determined from the equation (1) or (2).

Further, chromatic dispersion will be described. When the optical output signal S _OUT of the PAM 8 is transmitted through an optical fiber, the oscillation wavelength is limited due to the chromatic dispersion of the optical fiber. The group delay time τ due to chromatic dispersion at the transmission distance L of the optical fiber is expressed by the following equation.

Here, λ ₀ is the zero dispersion wavelength of the optical fiber, and S is the slope of the dispersion coefficient of the optical fiber with respect to the wavelength. For example, a typical single mode optical fiber has a zero dispersion wavelength near 1310 nm. Therefore, in the calculation example in the present embodiment, λ ₀ = 1310 nm. The value of S varies depending on the structure and wavelength of the optical fiber, but in an example of a general single mode optical fiber having a zero dispersion wavelength near 1310 nm, S = 8.7 × 10 ⁻¹⁴ s / km · nm ^2. This value is used in the calculation example of the embodiment.

Let λ _L = λ _M −Δλ and λ _U = λ _M + Δλ, where λ _L , λ _M , and λ _U are the lower limit value, the center value, and the upper limit value of the wavelength distribution range of a plurality of optical signals to be combined. The group delay time at each of λ _L and λ _U is obtained from the equation (4), and the difference between the group delay times of the optical signals generated by combining and output to the optical fiber, that is, the group delay time at λ _L and λ The difference Δτ with respect to the group delay time at _U is expressed by the following equation.
Δτ = 2LSΔλ (λ ₀ −λ _M ) (5)

Wavelength delay time difference Δτ is the optical output signal S _OUT appears as jitter between different optical components. For example, if the jitter is 1/10 or less of the reciprocal of the modulation rate BR, that is, it does not affect the discrimination of the transmission signal at the transmission distance L. Here, the margin for jitter due to group delay is given in both positive and negative directions. Therefore, the left side of equation (5) is
Δτ ≦ 2 / (10BR)
Thus, the range (and upper limit value) of Δλ in which the influence of chromatic dispersion is allowed can be obtained.

For example the modulation rate BR and 28GBaud, the lambda _M and 1305.0Nm, When 2km of L, [Delta] [lambda] becomes 4.1nm (5) below. That is, if the difference (2Δλ) between the wavelength λ ₁ , λ ₂ , and λ ₃ of the optical signal having the maximum wavelength λ _U and the minimum wavelength λ _L in the optical transmission module 1 is 8.2 nm or less. Thus, it is possible to avoid deterioration of information transmitted by the combined optical signal due to beat noise.

Therefore, in the case of BR = 28 GBaud, L = 2 km, considering both the wavelength condition for avoiding the influence of the beat noise and the wavelength condition for avoiding the influence of chromatic dispersion, for example, λ _C = λ _{When M} = 1305.0 nm, λ ₁ and λ ₂ may be set in the following ranges.
1300.9 nm ≦ λ ₁ ≦ 1304.84 nm
1305.16 nm ≦ λ ₂ ≦ 1309.1 nm

On the other hand, the optical signal of the optical signal source 13 having the wavelength λ ₃ is in a polarization state orthogonal to the optical signals of the optical signal sources 11 and 12 having the wavelengths λ ₁ and λ _{2. 1,} beat noise is combined with lambda ₂ of the optical signal components does not occur. Therefore, for λ _3, it is only necessary to consider the chromatic dispersion condition among the wavelength condition related to beat noise and the wavelength condition related to the chromatic dispersion of the optical fiber, and λ ₃ may be set in the following range.
1300.9 nm ≦ λ ₃ ≦ 1309.1 nm

When experiments were performed under the setting conditions for λ ₁ , λ ₂ , and λ ₃ , 28 Gbaud PAM8, that is, an error in which a floor does not occur up to 10 ⁻⁵ due to an offline error rate measurement after 2 km transmission in 84 Gb / s transmission. Rate characteristics could be obtained. With this characteristic, an error-free transmission can be obtained in an optical transmission module subjected to forward error correction (FEC).

FIG. 4 is a graph showing changes in Δλ in the expressions (3) and (5) when the center frequency is changed. The vertical axis is Δλ and the horizontal axis is the center frequency. In FIG. 4, conditions other than the center frequencies λ _C and λ _M are the same as those in the above calculation example. That is, BR = 28 GBaud, λ ₀ = 1310 nm, S = 8.7 × 10 ⁻¹⁴ s / km · nm ² , and L = 2 km. A curve 60 shows Δλ obtained from the equation (3), and a curve 61 shows Δλ obtained from the equation (5). A curve 60 represents a lower limit based on a condition relating to beat noise with respect to a difference Δλ between λ ₁ and λ ₂ with respect to the center frequency λ _C. On the other hand, the curve 61 represents the upper limit based on the condition relating to chromatic dispersion with respect to the difference Δλ of λ ₁ , λ ₂ , λ ₃ with respect to the center frequency λ _M.

For example, λ _1, λ _2, if the lambda ₁ of lambda ₃ is the minimum wavelength λ _L, λ ₂ is the maximum wavelength lambda _U, a λ _C = λ _M, Δλ is arbitrary position on the horizontal axis in FIG. 4 It can be set to a value within the range between the curve 60 and the curve 61. That is, in the optical transmission module 1, λ ₁ (≡λ _C −Δλ) is determined in which the wavelength of each optical signal source is determined corresponding to an arbitrary point (λ _C , Δλ) in the region sandwiched between the curves 60 and 61. , lambda ₂ and _{(≡λ C + Δλ),} by setting to the lambda ₃ that is set in the λ ₁ ≦ λ ₃ ≦ λ ₂ scope, beat noise and the wavelength of the optical output signal S _OUT of the optical transmission module 1 The influence of dispersion on signal quality can be kept within an allowable range.

Incidentally, (3), (5) satisfies the condition of formula, the optical transmission module 1 to fall within the allowable range the effect of beat noise and chromatic dispersion has on signal quality, the lambda ₃ lambda _{3 <lambda 1} or lambda Even if it is set in a range of ₂ <λ _3, it can be configured. However, construction of the λ ₁ ≦ λ ₃ ≦ λ ₂ described above, lambda _1, and the minimum value of the difference between the lambda ₂ can avoid the influence of beat noise, and sets the lambda ₃ between the lambda ₁ and lambda ₂ Therefore, it is preferable that the distribution range of the wavelengths λ ₁ , λ ₂ , and λ ₃ can be narrower than the configuration in which λ ₃ <λ ₁ or λ ₂ <λ ₃ .

Further, even if the optical signals to be combined with each other have the same wavelength, they are not affected by beat noise. Therefore, the wavelength of the optical signal from the optical signal sources 11 to 13 is reduced to two wavelengths, particularly as a configuration in which λ ₃ = λ ₁ or λ ₃ = λ ₂ among the above-described configurations in which λ ₁ ≦ λ ₃ ≦ λ _2. be able to. In this case, the modulator integrated semiconductor laser element 33 having the same configuration as any of the modulator integrated semiconductor laser elements 31 and 32 can be used. That is, by reducing the types of modulator integrated semiconductor laser elements used in the optical signal sources 11 to 13, for example, it is possible to reduce the design and manufacturing costs of the modulator integrated semiconductor laser elements.

As mentioned above, although the example of the specific wavelength was shown about communication with BR of 1 GB in the 1300 nm band regarding the optical transmission module 1, the present invention can also be applied to other wavelength bands and modulation rates BR. For example, in the case of BR = 56 GBaud, λ _C = 1305 nm, 2Δλ = 0.64 nm is obtained as the minimum wavelength difference between λ ₁ and λ ₂ related to beat noise from the equation (3), and the same conditions as in the above example From (5), 2Δλ = 4.1 nm is obtained as the maximum wavelength difference allowed for chromatic dispersion. From this calculation result, λ ₁ , λ ₂ , and λ ₃ in the optical transmission module 1 may be set within the following ranges, for example.
1302.95 nm ≦ λ ₁ ≦ 1304.68 nm
1305.32 nm ≦ λ ₂ ≦ 1307.05 nm
1302.95 nm ≦ λ ₃ ≦ 1307.05 nm

  As described above, in the present embodiment, it has been shown that the PAM8 signal can be generated by using wavelength multiplexing and polarization combining once each. Furthermore, it was shown that the PAM8 signal with excellent transmission characteristics can be generated by setting the wavelengths of the three optical signal sources based on the equations (3) and (5).

[Second Embodiment]
FIG. 5 is a schematic diagram showing a schematic configuration of the optical transmission module 1A according to the present embodiment. In the optical transmission module 1 </ b> A, the same components as those in the optical transmission module 1 are denoted by the same reference numerals.

Similar to the optical transmission module 1, the optical transmission module 1A combines the optical signals from the optical signal sources 11 to 13 by the combining unit 20A to generate one optical output signal S _OUT , specifically, a PAM8 signal. The combining unit 20 </ b> A is common to the combining unit 20 of the optical transmission module 1 in that it includes one wavelength multiplexer 21 and one polarization multiplexer 22. On the other hand, the combining unit 20A is different from the combining unit 20 in the order of wavelength multiplexing and polarization multiplexing, and the optical transmission module 1A is different from the optical transmission module 1 in this respect.

  Specifically, in the combining unit 20 </ b> A, the polarization multiplexer 22 performs polarization multiplexing on the optical signals from the optical signal sources 12 and 13, and the wavelength multiplexer 21 is generated by the polarization multiplexer 22. Wavelength multiplexing of the optical signal and the optical signal from the optical signal source 11 is performed.

The wavelengths λ ₁ , λ ₂ , and λ ₃ in the optical transmission module 1A can be determined in the same manner as in the optical transmission module 1.

[Third Embodiment]
Similar to the optical transmission module 1 described in the first embodiment, the optical transmission module 1B according to the third embodiment is, for example, an apparatus used for optical communication using an optical fiber as a transmission path. The difference between the optical transmission module 1B and the optical transmission module 1 is that the output optical signal S _OUT to the optical transmission line is not a PAM8 signal but a PAM16 signal. The optical transmission module 1B receives a PAM16 signal from four optical signals. Generate and output to the optical transmission line.

  FIG. 6 is a schematic diagram showing a schematic configuration of the optical transmission module 1B. In the optical transmission module 1A, the same components as those in the optical transmission module 1 are denoted by the same reference numerals, and the description thereof is basically omitted. Hereinafter, differences from the first embodiment will be mainly described.

  The optical transmission module 1B includes a first optical signal source 11, a second optical signal source 12, a third optical signal source 13, a fourth optical signal source 14, and a combining unit 20B. Although not shown in FIG. 6, like the optical transmission module 1, the optical transmission module 1 </ b> B includes a laser driving circuit 50 and a modulator control circuit 51. Similar to the optical signal sources 11 to 13 shown in FIG. 2, the optical signal source 14 is a modulator including a laser element driven by a laser driving circuit 50 and an EA modulator whose intensity modulation is controlled by a modulator control circuit 51. An integrated semiconductor laser element is provided.

The wavelength λ ₁ of the optical signal from the optical signal source 11 and the wavelength λ ₂ of the optical signal from the optical signal source 12 are different from each other, and here λ ₁ <λ ₂ . On the other hand, the optical signal source 11 and the optical signal source 12 output optical signals having the same polarization state (TE mode).

The wavelength λ ₃ of the optical signal from the optical signal source 13 and the wavelength λ ₄ of the optical signal from the optical signal source 14 are different from each other, and here λ ₃ <λ ₄ . On the other hand, the optical signal source 13 and the optical signal source 14 output optical signals in a polarization state (the TM mode) orthogonal to the optical signal sources 11 and 12.

The synthesizer 20B has two wavelength multiplexers 21a and 21b and one polarization multiplexer 22, and synthesizes the input optical signals from the optical signal sources 11, 12, 13, and 14 and outputs one output light. A signal S _OUT is generated.

  The wavelength multiplexer 21a combines the optical signal from the optical signal source 11 and the optical signal from the optical signal source 12, and the wavelength multiplexer 21b combines the optical signal from the optical signal source 13 and the optical signal from the optical signal source 14. Combine. The polarization multiplexer 22 multiplexes the output optical signal of the wavelength multiplexer 21a and the output optical signal of the wavelength multiplexer 21b.

The intensities of the optical signal sources 11 to 14 are set so that the intensity ratio of the optical signal components of the respective optical signal sources in the output optical signal S _OUT of the combining unit 20B is 1: 2: 4: 8. Hereinafter, the intensities of the optical signal components of the optical signal sources 11 to 14 in the output optical signal S _OUT are expressed as P ₁ , P ₂ , P ₃ , and P ₄ . Here, the intensity of each optical signal source when the value of the 1-bit modulated signal input to the EA modulator of each of the optical signal sources 11 to 14 is “1”, using the reference value P ₀ , P ₁ = 8P ₀ , P ₂ = 4P ₀ , P ₃ = 2P ₀ , P ₁ = P _0, and the values of P _{1 to} P ₄ when the value of the modulation signal is “0” are 0.

Table 2 shows combinations of values of 1-bit modulation signals inputted as control signals M _{1 to} M ₄ to the EA modulators of the optical signal sources 11 to 14 and output light of the combining unit 20B for the combinations. It is a table | surface which shows the relationship with the intensity _| strength of signal _SOUT . As shown in Table 2, it is possible to generate a PAM16 signal by combining 1-bit NRZ signals output from four optical signal sources and having different sizes.

The wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ of the optical signal combined by the optical transmission module 1B will be described.

Since the optical signals of the optical signal source 11 and the optical signal source 12 are wavelength-multiplexed by the wavelength multiplexer 21a, in order to avoid the influence of beat noise, as in the first embodiment, λ ₁ and λ differences based on equation (1) or (2) between the ₂ is provided.

Similarly, since the optical signals of the optical signal source 13 and the optical signal source 14 are wavelength-multiplexed with each other by the wavelength multiplexer 21b, λ ₃ and λ ₄ are calculated based on the equation (1) or (2). There is a difference between them.

Here, the output of the wavelength multiplexer 21a combined with the optical signals of the optical signal sources 11 and 12 and the output of the wavelength multiplexer 21b combined with the optical signals of the optical signal sources 13 and 14 are further combined. However, the multiplexing is polarization multiplexing by the polarization multiplexer 22. That is, since the polarization states of the optical signal components of the optical signal sources 11 and 12 and the optical signal components of the optical signal sources 13 and 14 input to the polarization multiplexer 22 are orthogonal to each other, no beat noise is generated. . Therefore, it is not necessary to provide a wavelength difference satisfying the expressions (1) and (2) between λ ₁ and λ ₂ and λ ₃ and λ ₄ .

On the other hand, the distribution range of the wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ needs to be set so that Δτ given by the equation (5) is less than the allowable value in order to avoid the influence of chromatic dispersion.

For example, the conditions shown in the first embodiment, that is, BR = 28 GBaud, λ ₀ = 1310 nm, S = 8.7 × 10 ⁻¹⁴ s / km · nm ² , L = 2 km, λ _C = λ _M = 1305 About the settable range of λ ₁ and λ ₂ at.
1300.9 nm ≦ λ ₁ ≦ 1304.84 nm
1305.16 nm ≦ λ ₂ ≦ 1309.1 nm
Similarly, for the settable ranges of λ ₃ and λ ₄ ,
1300.9 nm ≦ λ ₃ ≦ 1304.84 nm
1305.16 nm ≦ λ ₄ ≦ 1309.1 nm
It becomes.

Experiments were performed under the setting conditions for λ ₁ , λ ₂ , λ ₃ , and λ _4. Up to 2 × 10 ⁻⁵ by measuring the error rate offline after 2 km transmission in 28 Gbaud PAM16, that is, 112 Gb / s transmission. An error rate characteristic that does not generate a floor can be obtained. With this characteristic, an error-free transmission can be obtained in an optical transmission module subjected to forward error correction (FEC).

As described above, there is no need to provide a wavelength difference between λ ₁ and λ ₂ and λ ₃ and λ _4. Therefore, in order to reduce the influence of chromatic dispersion, λ ₁ = λ ₃ and λ ₂ = Λ ₄ is preferable.

  In the optical transmission module 1B described above, the optical signal sources 13 and 14 are configured to generate TM mode optical signals. This configuration is possible by providing the polarization rotation plate 52 in each of the optical signal sources 13 and 14 as in the optical signal source 13 of the first embodiment. Here, a polarization rotation plate 52 is inserted between the wavelength multiplexer 21 b and the polarization multiplexer 22, and the optical signal sources 13 and 14 output TE mode light as the optical signal sources 11 and 12. This configuration is equivalent to the configuration in which the polarization rotation plate 52 is provided in the optical signal sources 13 and 14 described above with respect to the characteristics of the present invention that generates the PAM signal while avoiding the effects of beat noise and chromatic dispersion.

[Fourth Embodiment]
FIG. 7 is a schematic diagram showing a schematic configuration of an optical transmission module 1C according to the fourth embodiment. The optical transmission module 1C is a device that generates a PAM16 signal, similar to the optical transmission module 1B of the third embodiment. In the optical transmission module 1C, the same reference numerals are given to the same components as the optical transmission module 1B. The description will be basically omitted, and the description below will focus on differences from the optical transmission module 1B.

The wavelength λ ₁ of the optical signal from the optical signal source 11 and the wavelength λ ₃ of the optical signal from the optical signal source 13 are different from each other. On the other hand, the optical signal source 11 and the optical signal source 13 output optical signals having the same polarization state (TE mode).

The wavelength λ ₂ of the optical signal from the optical signal source 12 and the wavelength λ ₄ of the optical signal from the optical signal source 14 are different from each other. On the other hand, the optical signal source 12 and the optical signal source 14 output optical signals in a polarization state (the TM mode) orthogonal to the optical signal sources 11 and 13.

Optical transmission module 1C has a means for generating one of the output optical signal _{S OUT} from the optical signal of the optical signal source 11 to 14, a composition unit 20C instead of the combining section 20B of the optical transmission module 1B. The combining unit 20 </ b> C has two polarization multiplexers 22 a and 22 b and one wavelength multiplexer 21.

  The polarization multiplexer 22 a combines the optical signal of the optical signal source 11 and the optical signal of the optical signal source 12, and the polarization multiplexer 22 b is the optical signal of the optical signal source 13 and the light of the optical signal source 14. Combine the signal. The wavelength combiner 21 combines the output optical signal of the polarization combiner 22a and the output optical signal of the polarization combiner 22b.

Since the optical signals of the optical signal source 11 and the optical signal source 12 are combined with each other by the polarization multiplexer 22a, no beat noise is generated. Therefore, it is not necessary to provide a wavelength difference satisfying the expressions (1) and (2) between λ ₁ and λ ₂ .

Similarly, since the optical signals of the optical signal source 13 and the optical signal source 14 are polarized and multiplexed with each other by the polarization multiplexer 22b, the equations (1) and (2) are set between λ ₃ and λ _4. There is no need to provide a wavelength difference that satisfies the equation.

On the other hand, in the wavelength multiplexer 21, the TE mode components of the output optical signals of the polarization multiplexers 22a and 22b are wavelength-multiplexed, and the TM mode components are wavelength-multiplexed. Specifically, the optical signal component of the optical signal source 11 and the optical signal component of the optical signal source 13 are combined in wavelength, and the optical signal component of the optical signal source 12 and the optical signal component of the optical signal source 14 are combined in wavelength. Waved. Therefore, in order to avoid the influence of beat noise, a difference is provided between λ ₁ and λ ₃ based on the equation (1) or (2), and similarly between λ ₂ and λ ₄ (1 ) Or (2) based on the difference.

For avoiding the influence of chromatic dispersion, as in the third embodiment, the distribution range of wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ is limited so that Δτ given by equation (5) is less than the allowable value. There is a need to.

As described above, there is no need to provide a wavelength difference between λ ₁ and λ ₂ and λ ₃ and λ _4. Therefore, in order to reduce the influence of chromatic dispersion, λ ₁ = λ ₂ and λ ₃ = λ ₄ is preferred.

[Fifth Embodiment]
FIG. 8 is a schematic diagram showing a schematic configuration of an optical transmission module 1D according to the fifth embodiment. The optical transmission module 1D is a device that generates a PAM16 signal, similar to the optical transmission module 1B of the third embodiment. In the optical transmission module 1D, the same components as those of the optical transmission module 1B are denoted by the same reference numerals. The description will be basically omitted, and the description below will focus on differences from the optical transmission module 1B.

The wavelengths λ ₁ , λ ₂ , and λ ₃ of the optical signals of the optical signal sources 11 to 13 are different from each other, and here, λ ₁ <λ ₂ <λ ₃ . On the other hand, the optical signal sources 11 to 13 output optical signals having the same polarization state (TE mode).

  The optical signal source 14 outputs an optical signal having a polarization state (TM mode) orthogonal to the optical signal sources 11 to 13.

Optical transmission module 1D has a means for generating one of the output optical signal _{S OUT} from the optical signal of the optical signal source 11 to 14, the combining unit 20D instead of the combining section 20B of the optical transmission module 1B. The synthesizer 20D includes one polarization multiplexer 22 and two wavelength multiplexers 21a and 21c.

  The polarization multiplexer 22 multiplexes the optical signal from the optical signal source 13 and the optical signal from the optical signal source 14. The wavelength multiplexer 21a combines the optical signal of the optical signal source 11 and the optical signal of the optical signal source 12, and the wavelength multiplexer 21c and the output optical signal of the polarization multiplexer 22 and the wavelength multiplexer. The output optical signal 21a is multiplexed.

Since each of the optical signal an optical signal source 11 and the optical signal source 12 to the wavelength multiplexing each other at a wavelength multiplexer 21a, in order to avoid the influence of the beat noise, between the lambda ₁ and lambda ₂ (1) formula Or a difference is provided based on (2) Formula.

Since the optical signals of the optical signal source 13 and the optical signal source 14 are combined with each other by the polarization multiplexer 22, no beat noise is generated. Therefore, it is not necessary to provide a wavelength difference satisfying the expressions (1) and (2) between λ ₃ and λ ₄ .

On the other hand, in the wavelength multiplexer 21c, the TE mode component of the output optical signal of the polarization multiplexer 22 and the output optical signal of the wavelength multiplexer 21a are wavelength-multiplexed. Specifically, the optical signal components of the optical signal sources 11 to 13 are wavelength-multiplexed. Therefore, it is necessary to provide a difference between λ ₃ and λ ₁ , λ ₂ that can avoid the influence of beat noise. Here, λ ₁ <λ ₂ <λ ₃ and a necessary difference is provided between λ ₁ and λ ₂ with respect to the multiplexing at the wavelength multiplexer 21a, so that the multiplexing at the wavelength multiplexer 21c. Regarding the wave, a difference is provided between λ ₂ and λ ₃ based on the equation (1) or (2).

Regarding avoidance of the influence of chromatic dispersion, the distribution of the wavelengths λ ₁ , λ ₂ , λ ₃ , λ ₄ so that Δτ given by the equation (5) is less than the allowable value, as in the third and fourth embodiments. Limit the range.

Note that λ ₄ can be shared with any _one of λ ₁ , λ ₂ , and λ ₃ .

[Modification]
In the first embodiment, the ratio of the intensity of the optical signal components of the wavelengths λ ₁ , λ ₂ , and λ ₃ in the optical output signal S _OUT of the optical transmission module 1 is 4: 2: 1, and the TM mode optical signal Although an example in which the component intensity is P ₀ has been shown, in the first and second embodiments, the correspondence relationship between the wavelengths λ ₁ , λ ₂ , λ ₃ and the optical signal intensities P ₀ , 2P ₀ , 4P _0. , And the intensity of the optical signal component in the TM mode may be any of P ₀ , 2P ₀ , and 4P ₀ .

Similarly, in the third embodiment, the ratio of the intensity of the optical signal components of the wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ in the optical output signal S _OUT of the optical transmission module 1B is 8: 4: 2: 1. In the third to fifth embodiments, the correspondence relationship between the wavelengths λ ₁ , λ ₂ , λ ₃ , λ ₄ and the optical signal intensities P ₀ , 2P ₀ , 4P ₀ , 8P ₀ is shown. Is optional. Moreover, the intensity of the light signal components of the TM mode in the optical output signal _{S OUT} of the optical transmission module _{1B~1D P 0, 2P 0, 4P} 0, or also arbitrary and any 8P _0.

  In each of the above-described embodiments, the configuration in which the EA modulator is used as the light modulation means has been described, but it goes without saying that the same effect can be obtained by using an MZ modulator instead.

  Each of the optical components constituting the above-described optical transmission modules 1 and 1A to 1D may be partly or entirely integrated on a semiconductor substrate by monolithic integration or lamination. , All may be discrete components. In the above description, the polarization rotation plate 52 is a component of the optical signal source 13, but the polarization rotation plate 52 may be disposed outside the optical signal source.

As described above, the optical transmission module that generates the PAM8 signal from the three optical signal sources and the optical transmission module that generates the PAM16 signal from the four optical signal sources have been described. Here, the present invention generates 2 ⁿ level PAM signals from first to nth optical signal sources (n is a natural number of 3 or more) that outputs optical signals that are binary intensity modulated with different amplitudes. It can be applied to an optical transmission module.

  1, 1A, 1B, 1C, 1D optical transmission module, 11, 12, 13 optical signal source, 20 combiner, 21, 21a-21c wavelength combiner, 22, 22a, 22b polarization combiner, 31, 32 , 33 Modulator integrated semiconductor laser element, 31a, 32a, 33a DFB laser, 31b, 32b, 33b EA modulator, 50 laser drive circuit, 51 modulator control circuit, 52 polarization rotation plate.

Claims (12)

First to n-th optical signal sources (n is a natural number of 3 or more) for outputting optical signals that are binary intensity modulated with different amplitudes;
A combining unit that combines a plurality of input optical signals to generate one output optical signal;
The synthesis unit is
A wavelength multiplexer for wavelength-multiplexing a plurality of input optical signals having different wavelengths while maintaining the polarization state;
A polarization multiplexer that combines a pair of input optical signals having polarization states orthogonal to each other;
Have
Combining input optical signals from the first to nth optical signal sources to generate an output optical signal that is pulse amplitude modulated to 2 ⁿ levels;
An optical transmission module characterized by the above. The optical transmission module according to claim 1,
Wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ) of the optical signals of the two optical signal sources that are combined with each other by the wavelength multiplexer are the light velocity C ₀ and the modulation of the optical signal. The rate is BR

An optical transmission module characterized by satisfying. The optical transmission module according to claim 1 or 2,
The upper limit wavelength and the lower limit wavelength of the first to nth optical signal sources are λ _M + Δλ and λ _M −Δλ, respectively.
A difference in group delay time between the light of the upper limit wavelength and the light of the lower limit wavelength in the output optical signal of the optical transmission module after transmitting the distance L through the optical transmission line is Δτ,
The upper limit wavelength and the lower limit wavelength are:
Δτ = 2LSΔλ (λ ₀ −λ _M )
(Where λ ₀ is the zero dispersion wavelength of the optical transmission line, and S is the slope of the wavelength dispersion coefficient of the optical transmission line with respect to the wavelength.)
Satisfying the condition that Δτ given by is less than or equal to a predetermined allowable value for a predetermined distance L;
An optical transmission module characterized by the above. In the optical transmission module according to any one of claims 1 to 3,
N is 3;
The first optical signal source and the second optical signal source output optical signals having different wavelengths and the same polarization state;
The third optical signal source outputs an optical signal in a polarization state orthogonal to the first optical signal source and the second optical signal source;
The wavelength multiplexer combines the optical signal of the first optical signal source and the optical signal of the second optical signal source,
The polarization multiplexer combines the output optical signal of the wavelength multiplexer and the optical signal of the third optical signal source;
An optical transmission module characterized by the above. In the optical transmission module according to any one of claims 1 to 3,
N is 3;
The first optical signal source and the second optical signal source output optical signals having different wavelengths and the same polarization state;
The third optical signal source outputs an optical signal in a polarization state orthogonal to the first optical signal source and the second optical signal source;
The polarization multiplexer combines the optical signal of the second optical signal source and the optical signal of the third optical signal source,
The wavelength multiplexer multiplexes the output optical signal of the polarization multiplexer and the optical signal of the first optical signal source;
An optical transmission module characterized by the above. The optical transmission module according to claim 4 or 5,
An optical transmission module, wherein an intensity ratio of an optical signal component of each optical signal source in an output optical signal of the combining unit is 1: 2: 4. In the optical transmission module according to any one of claims 1 to 3,
N is 4;
The first optical signal source and the second optical signal source output optical signals having different wavelengths and a first polarization state;
The third optical signal source and the fourth optical signal source output optical signals having different wavelengths and a second polarization state orthogonal to the first polarization state;
The wavelength multiplexer includes a first wavelength multiplexer that multiplexes an optical signal of the first optical signal source and an optical signal of the second optical signal source, and an optical signal of the third optical signal source. A second wavelength multiplexer for multiplexing the optical signal of the fourth optical signal source,
The polarization multiplexer combines the output optical signal of the first wavelength multiplexer and the output optical signal of the second wavelength multiplexer;
An optical transmission module characterized by the above. In the optical transmission module according to any one of claims 1 to 3,
N is 4;
The first optical signal source and the third optical signal source output optical signals having different wavelengths and a first polarization state;
The second optical signal source and the fourth optical signal source output optical signals having different wavelengths and a second polarization state orthogonal to the first polarization state;
The polarization multiplexer includes a first polarization multiplexer that combines the optical signal of the first optical signal source and the optical signal of the second optical signal source, and the light of the third optical signal source. A second polarization multiplexer that combines the signal and the optical signal of the fourth optical signal source,
The wavelength multiplexer multiplexes the output optical signal of the first polarization multiplexer and the output optical signal of the second polarization multiplexer;
An optical transmission module characterized by the above. In the optical transmission module according to any one of claims 1 to 3,
N is 4;
The first to third optical signal sources output optical signals having different wavelengths;
The fourth optical signal source outputs an optical signal having a polarization state orthogonal to the polarization state of the third optical signal source;
The polarization multiplexer combines the optical signal of the third optical signal source and the optical signal of the fourth optical signal source,
The wavelength multiplexer includes a first wavelength multiplexer that multiplexes an optical signal of the first optical signal source and an optical signal of the second optical signal source, and an output optical signal of the polarization multiplexer. And a second wavelength multiplexer for multiplexing the output optical signal of the first wavelength multiplexer,
An optical transmission module characterized by the above. In the optical transmission module according to any one of claims 1 to 9,
One optical signal source of two optical signal sources that output optical signals in polarization states that are orthogonal to each other includes polarization rotation means that changes the output optical signal to a polarization state orthogonal to the other optical signal source. And an optical transmission module. In the optical transmission module according to any one of claims 1 to 10,
The optical transmission module, wherein the optical signal source is a semiconductor optical device having a distributed feedback laser and an electroabsorption modulator. In the optical transmission module according to any one of claims 4 to 6,
The modulation rate BR of the optical signal is 28 gigabaud (GBaud),
When the transmission distance L in the optical transmission line is 2 kilometers (km), the allowable value of the difference Δτ in the group delay time between the upper limit wavelength and the lower limit wavelength of the optical signal is set to 1 / reciprocal of the reciprocal of the modulation rate BR. 5 and
When the optical transmission line is an optical fiber having a zero dispersion wavelength λ ₀ of 1310 nanometers (nm) and a slope S of the wavelength dispersion coefficient with respect to the wavelength being 8.7 × 10 ⁻¹⁴ seconds / km · nm ^2. ,
The wavelengths λ ₁ , λ ₂ , λ _{3 of} each of the first to third optical signal sources are
1300.9 nm ≦ λ ₁ ≦ 1304.84 nm
1305.16 nm ≦ λ ₂ ≦ 1309.1 nm
1300.9 nm ≦ λ ₃ ≦ 1309.1 nm
Meeting,
An optical transmission module characterized by the above.

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